1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor and, more particularly, to a method for manufacturing a thin film transistor used for a large-sized display device.
2. Description of Related Art
Generally, a thin film transistor liquid crystal display device (TFT-LCD) mainly consists of a TFT array substrate, a color filter array substrate, and a liquid crystal (LC) layer, wherein the TFT array substrate includes plural pixel structures that consist of plural transistors in array arrangement and plural pixel electrodes. Each of the pixel electrodes corresponds to every transistor. The above-mentioned TFT mainly comprises a gate, a semiconductor layer, a source, a drain, and a channel. Mostly, the TFT is a switch component adopted for a liquid crystal display pixel unit.
Currently, the development object of LCDs tends toward a large size, high brightness, high contrast, a wide angle of view, and high color saturation. As larger and larger panels are manufactured, the current Ion (i.e. the current as turning on the TFT) produced from every TFT is required to become higher and higher accordingly. In order to satisfy the requirement of the large-sized LCD panel, the direct way for promoting the current Ion of the TFT is to increase the ratio (W/L) of the channel width (W) vs. the channel length (L).
However, the resolution of exposure equipment now in use is about 4 μm. After the subsequent etching processes are performed, the channel length (L) in the TFT becomes in the limit of about 4.5 μm to 5 μm. Therefore, the way now in use for promoting the current Ion of the TFT can just be achieved by changing the channel width (W). For example, through designing U-type or double-U-type source/drain (S/D), the channel width (W) can be increased.
Nevertheless, if the source/drain (S/D) is designed in U-type or double-U-type, it still enlarges the area of the TFT component. Hence, the aperture ratio and the transmittance of the display device are both decreased, and the image quality of the display device is debased. Additionally, the desired pattern can not be obtained in the large-sized panel through being exposed by only one mask. In accordance with different process generations, the desired pattern generally is completed through three or several ten times exposure processes. Therefore, in order to avoid existence of heterogeneous images, the alignment of the exposure equipment is required to be extremely accurate. The aforementioned problems of the exposure processes in use have remained unsolved for some considerable time.